This disclosure relates to methods and apparatus useful for operating on a multiplicity of diversely (or, as is commonly known in the art, diversity) received replicas of a signal originating in the same remote transmitter to effect a process of digital detection that combines estimates of essential characteristics of said replicas to achieve a lower average probability of bit error than is possible with any of the replicas separately. In essence, the so-called diversity combining digital detection technique according to this invention compares the diversely received replicas of the signal on the basis of quantitative measures of wanted signal and attendant unwanted interfering signal and noise power or energy, and, after compensating for differences in their times on arrival, employs the results to effect a final bit-by-bit decision process based on selected groupings of preliminary signal replica-by-replica bit decisions, or on controlled weighted combinations of the video-band bit-stream waveforms of the various signal replicas.
The diversity receiving and combining technique according to this invention operates on a set of diversely received replicas of a digitally modulated sinusoidal prime-carrier or subcarrier, illustrated in this disclosure in terms of binary code frequency or phase modulated sinusoid, commonly designated in the art as PCM/FM for pulse-code modulation (PCM) on a frequency-modulated carrier, or as PCM/.phi.M for PCM on a phase modulated carrier, and delivers an output PCM stream with the least bit error rate (BER) or probability possible from the combination of replicas in the set. In order to achieve this result, said combining technique comprises three essential operations:
1) Measurement of Signal Quality (SQM) in terms of a measurable that is reliably/consistently and monotonically indicative of the bit error rate;
2) Time Alignment of (differently delayed) Replicas of the PCM Bit Stream;
3) Merging of the Time-Aligned Replicas of the Bit Stream into one bit stream characterized by the least error rate achievable from the combination.
Candidate measurables considered for signal quality evaluation include:
i) Direct indicators:
Bit Error Rate (BER) PA1 Decoded Data Quality Measurables PA1 Pre-detection Signal-to-Noise Ratio (IF SNR) PA1 Carrier Baseband SNR PA1 PCM Videoband SNR PA1 Bit Waveform Characteristics (Bit Time or/and Amplitude Jitter) PA1 AGC PA1 Offband Noise PA1 Noise at PCM Spectral Zeros
ii) Indirect indicators:
The indirect indicators are readily discarded as being not reliably indicative of the BER, and hence inadequate for formulating a consistent diversity signal combining/selection process.
It is therefore an object of this invention to provide a diversity signal combining/selection method based on a signal quality indicator that is reliably monotonically indicative of the bit error rate (BER).
Among the direct indicators,
Direct measurement/counting of bit error rate (BER) requires a priori knowledge of the correct prefixed bits (i.e., parity bits, word sync bits, frame sync bits, ID bits). As overhead bits, prefixed bits reduce the efficiency of telemetry link capacity usage. Therefore, the ratio of a priori known prefixed bits to total number of bits in a given period of time is usually made as small as possible. This ratio, together with the desire to achieve a BER on the order of 10.sup.-6, combines to require inordinately excessive measurement times, completely incompatible with an essential requirement for on-line, "real-time" evaluation of signal quality.
It is therefore a further object of this invention to base the diversity signal combining/selection on a method of signal quality evaluation that enables reliable inference of the value of average BER without requiring direct counting of bits in error in a test code word, whether separate, or present in the actual PCM stream as for example for frame synchronization purposes.
The direct measurement of SNR of a PCM bit stream breaks down under conditions of relatively poor signal because one of its key component operations becomes erratic when the pulse time jitter becomes a significant fraction of the pulse width. This is PCM videoband SNR and in essence is also the carrier baseband SNR in PCM/FM and PCM/.phi.M. Bit waveform characteristics, including bit time jitter and bit amplitude fluctuations, provide alternative ways to measure the bit stream SNR. These measures of signal quality have (with some simplifying assumptions) a limited range in which they can be considered correct and sufficient indicators of BER value.
It is therefore yet a further object of the present invention to base the evaluation of signal quality on robust measurements (i.e., measurements that are insensitive to signal waveform distortion in the transmission medium and/or in the receiving equipment, as well as to the noise characteristics relative to the signal) over a wide dynamic range of signal relative to noise energy.
Bit error probability can be shown to be expressible, for IF SNR of 4 dB or greater, as EQU P.sub.e .perspectiveto.(1/2)exp{-(S/N).sub.if /2}, for PCM/FM EQU .perspectiveto.(1/2.sqroot..pi.).{1/.sqroot.(S/N).sub.if }exp{-(S/N).sub.if }, for PCM/.phi.M
Thus, in either case, the BER can be computed directly from the measured value of IF SNR, and is a monotonically decreasing function of it. Moreover, if a subset of the received signal replicas shows one of the binary symbols in a particular bit time slot, and the remaining replicas show the other binary symbol, then the probability that either of the two subsets will be jointly in error is given by the product of the bit error probabilities of the members of the subset. Therefore, if we add the measured values of IF SNR for each subset, the one with the greater sum will be less likely to be in error. As a result, a bit-by-bit decision based on the greater of the sums of IF SNR's during each bit time will guarantee a merged bit stream with the lowest achievable BER from the combination of diversely received replicas of the signal. For these reasons, and for the all-important reason that the IF SNR is readily and quickly measurable, the IF SNR has been chosen here as the prime indicator of the BER in the combiner method and apparatus of this invention. The value of P.sub. e inferred from a measurement of (S/N).sub.if applies to all bit decisions within the time interval taken by each evaluation of (S/N).sub.if.
It is therefore yet a further object of this invention to provide a method of measuring pre-demodulation prime-carrier or subcarrier signal-to-noise ratio, (S/N).sub.if, that is insensitive to frequency and/or amplitude modulation.
It is yet a further object of this invention to provide a diversity combining decision process that bases the final bit-by-bit decision on a set of decision indices, one corresponding to each code alphabet letter, each defined during each bit-symbol waveform time slot as a function solely of a subset of measured (S/N).sub.if 's of replicas of the signal.
Prior art PCM stream time-phase alignment techniques are generally based on tapped digital delay lines, which introduce quantized compensatory delays to bring corresponding frame sync pulses into very approximate time coincidence, to within a .+-.1/(2M) bit transient at each switchover between bit streams. The selection of M can be made to hold the transient maximum amplitude to a tolerable value if the switchovers are postulated to have some minimum period. Since M represents the number of clock phases necessary and each clock phase requires gating and gating control signals, the hardware complexity increases as M is increased. The storage capacity required is given by the product (Bit Rate).times.(Number of Bit Streams).times.(Maximum Delay Difference, or Delay Spread). The power required to keep this number of bits moving at the bit rate through a dynamic storage structure such as a shift register is, in itself, significant.
It is therefore a further object of this invention to provide a time-phase alignment method that is not dependent on quantized-delay increments to achieve near-perfect alignment of the PCM streams.
These and other objects and features of this invention will become apparent from the claims, and from the following description when read in conjunction with the accompanying drawings.